Lithium batteries have become established as energy stores above all for applications in portable electronics (laptops, mobile telephones), because of their high energy density and power density in comparison to other battery types. A distinction is made between primary lithium batteries, which are non-rechargeable batteries having mostly lithium metal anodes, and secondary systems, in other words rechargeable batteries.
Both battery types contain anhydrous liquid or gel-like ion-conductive electrolytes, in which supporting electrolytes, for example LiPF6, LiBF4, lithium imides, lithium methides or lithium borate salts, for example lithium bis(oxalato)borate (LiBOB, corresponding to Li[B(C2O4)2]), are present in dissolved form.
In comparison to lithium element fluorides such as LiPF6 or LiBF4, lithium borate salts such as LiBOB bring about a significant improvement in cycle stability and safety properties in secondary lithium batteries (Cox, S. S. Zhang, U. Lee, J. L. Allen, T. R. Jow, J. Power Sources 46, 2005, 79-85). This is due to a modified form of protective coating formation on the carbon anode of a lithium battery: borate electrolytes give rise to the formation of a thin, very stable Li+-conductive coating on this anode, which is stable even at elevated temperatures and thus prevents dangerous decomposition reactions between the charged anode and the electrolyte, for example (J.-C. Panitz, U. Wietelmann, M. Wachtler, S. Ströbele, M. Wohlfahrt-Mehrens, J. Power Sources 153, 2006, 396-401; Chemetall brochure 2005). The improvements to the protective coating brought about by borate salts offer users new possibilities for electrolyte formulation. For instance, the difficult-to-handle ethylene carbonate (1,3-dioxolan-2-one), for example, can be abandoned in favour of propylene carbonate (4-methyl-1,3-dioxolan-2-one) (K. Xu, S. Zhang, R. Jow, J. Power Sources 143, 2005, 197-202).
Lithium borate salts for example having the general formulae I or II are used:

L is a chelating agent having two terminal oxygen atoms with the general formula
wherein:                Y1 and Y2 together denote O, where m=0 or 1, n=0 or 1, o=0 and R1 and R2 independently of one another denote H, F, Cl, Br, OR (R=alkyl) or R′ (alkyl), or        Y1, Y2, Y3, Y4 independently of one another each denote OR (R=alkyl), H, F, Cl, Br, R (alkyl), where m=0 or 1, n=0, o=1, or        Y1, C1, Y3 and C2 are members of a 5- or 6-membered aromatic or heteroaromatic ring (with N, O or S as heteroelement), which can optionally be substituted with alkyl, alkoxy, carboxy or nitrile, wherein Y2 and Y4 are omitted, with n=0 and m=0 or 1, o=1.        
Lithium borate salts are generally produced by reacting an oxidic boron compound (for example boric acid, boron oxide or a boric acid ester) with oxalic acid or an oxalic acid salt or a fluoride donor, for example BF3, and optionally further dihydroxy compounds, for example dicarboxyl compounds, diphenols, and a lithium raw material, for example lithium carbonate, lithium hydroxide, lithium alcoholate or similar.
The commonest method of producing bis(chelato)borates of type I involves suspending the components in a solvent and separating off the water azeotropically (E. Bessler and J. Weidlein, Z. Naturforsch. 37b, 1020-1025, 1982).

Suitable solvents are those which form an azeotrope with water, for example saturated or aromatic solvents such as heptane, octane, toluene or cumene.
In a variant the alkali metal can also be incorporated via the lithium salt of the ligand (LiHL or Li2L) or a metal borate, for example LiBO2, for example:

A further production possibility is to react a metal tetraalkoxyborate M[B(OR)4] with two equivalents of the ligands in an organic solvent (DE-C-19829030), for example:
where R is an alkyl radical, for example H3C or C2H5.
The alcohol itself (formed in the reaction, ROH), for example methanol or ethanol, or an aprotic, polar solvent, for example acetonitrile, can be used as the organic solvent.
Finally, the production of LiBOB in homogeneous aqueous solution by reaction according to (1), (2), (3) or (4) and isolation in solid, anhydrous form after total evaporation and vacuum drying is known. The disadvantage of this process is that the space-time yield is relatively low. For instance, in DE-C-19829030, Example 1, only 185 g of product are obtained from approx. 3.1 kg of reaction solution.
DE-A-10108608 discloses the synthesis of alkali metal bis(chelato)borates by means of the reactions listed above without addition of solvents in the heterogeneous phase and removal of the water formed during the reaction. This process has the disadvantage of relatively poor drying results. For instance, DEA-10108608, Example 1, discloses a product having a water content of 0.4%. This water content is well above the values required for supporting electrolytes for batteries.
Compounds having the general formula II can be produced by reacting boron trifluoride with lithium salts. For example, lithium difluorooxalatoborate (LiDFOB) is produced by reacting BF3.Et2O (a complex of boron trifluoride with diethyl ether as solvate) and Li2C2O4 (S. S. Zhang, Electrochem. Commun. 8 (2000, 1423-1428):

Many supporting electrolytes decompose more or less quickly in the presence of protic compounds such as water, in the following manner for example:

The gaseous products formed during the hydrolysis of fluorine-containing supporting electrolytes, for example HF and POF3, are highly caustic and damaging to the other battery components, for example the cathode materials. Thus HF leads to the disintegration of manganese spinels, for example, and destroys the top coating on the electrode materials, which is important for a long operating life. The cycle stability of secondary batteries is impaired as a consequence. Borate electrolytes are also sensitive to water. In this case hydrolysis products, some of them insoluble, are formed, which likewise impair the functional properties of the batteries. Hydrolysis products such as boric acid or oxalic acid are acid-corrosive and similarly impair the formation of the top coating on the cathode or anode materials.
It is therefore essential to use products with the lowest possible water and acid contents for the production of battery electrolytes if batteries having long-term cycle stability are required.
The removal of water and/or acids can take place at the liquid electrolyte stage. DE-A-10049097 discloses the separation of water and protic contaminants from an organic liquid electrolyte by bringing it into contact with insoluble alkali-metal hydrides and separating off the insoluble secondary reaction products. The disadvantage of the process described is that the drying times are relatively long and the amounts of drying agent to be used are very high; thus approx. 0.4 to 6 g of lithium hydride are used per kg of electrolyte solution, corresponding to about 2 to 25 g per kg of lithium borate salt content.
In order to keep the amount of purification work at the end of the electrolyte production process as low as possible, it is necessary to use a lithium borate salt which is already largely dry and free from acid.